1. Field of the Invention
The present invention relates to a process for purifying an aqueous alkali chloride solution (hereinafter referred to as a "salt solution") for cation-exchange membrane electrolysis of alkali chloride. More specifically the present invention relates to a process for removing accumulated chlorate salts in a salt solution circulating in an electrolysis system.
2. Description of the Related Art
Several processes have been disclosed regarding the removal of chlorate salts from a salt solution. In a typical process, chlorate salts in a salt solution are decomposed by the addition of hydrochloric acid through the reactions shown below and removed therefrom. EQU ClO.sub.3 +2 HCl.fwdarw.ClO.sub.2 +1/2 Cl.sub.2 +Cl+H.sub.2 O(1)
or EQU ClO.sub.3 +6 HCl.fwdarw.3 Cl.sub.2 +Cl+3 H.sub.2 O (2)
This process is disadvantageous because the reaction of hydrochloric acid with chlorate salts is slow and therefore, removal of the chlorate salts from the aqueous salt solution is a lengthy process.
Accordingly, further improvements have been disclosed for the quick removal of chlorate salts from a salt solution: for example, a method of saturating the salt solution with alkali chloride and subsequently adding hydrochloric acid (Japanese Patent Application Laid-Open No. Sho 59-20483); and a method of maintaining the concentration of hydrochloric acid in the salt solution at a level higher than 150 g/l (Japanese Patent Application Laid-Open No. Sho 57-191225). However, in the former method wherein a salt solution is saturated with an alkali chloride and hydrochloric acid is then added to the solution, problems are likely to be encountered as the result of the deposition of salts on the addition of hydrochloric acid. In the latter method wherein the hydrochloric acid concentration is maintained at a level higher than 150 g/l, the resulting hydrochloric acid concentration after decomposition of the chlorate salts is excessively high. Thus, these methods are disadvantageous because they require the posttreatment addition of chemicals as well as complicated post-treatment steps such as adding neutralizing alkali to lower the hydrochloric acid concentration and the treating salt deposits, respectively.
In a process within the present invention, chlorate salts in a salt solution are decomposed by the addition of a relatively small amount of hydrochloric acid with the resulting formation of chlorine dioxide as shown in Formula (1) above. Japanese Patent Application Laid-Open No. Sho 61-01402A discloses a method for recovering chlorine gas from chlorine dioxide formed in the decomposition and removal of chlorate salts in a salt solution in which the resulting chlorine dioxide is further decomposed into chlorine and oxygen by heating. This method, however, is disadvantageous because it requires the complicated steps of decomposing chlorine dioxide to produce chlorine and oxygen and then separating the two gases.
Another method for removing chlorate salts from a salt solution is disclosed in Japanese Patent Application Laid-Open No. Sho 56-163286. In this method, a catalyst layer is provided in a salt solution which is circulated via a pipeline which feeds the salt solution to an alkali chloride electrolysis cell of an membrane ion-exchanger. The chlorate salt is decomposed and then removed by introducing hydrogen or a hydrogen-containing gas. The problems associated with this method include an increase in impurities as a result of elution of the catalyst and an increase in cost due to the complexity of the process.
Still another method for removing chlorate salts from an aqueous salt solution is disclosed in Japanese Patent Application Laid-Open No. Sho 51-144399. In this method, aliquots of the circulating salt solution are removed and cooled in order to crystallize out and thereby isolate chlorate salt. The problems associated with this method include the relatively large amount of chlorate salt remaining in the salt solution, the complexity of the process, and the high cost of cooling.
In other known methods, chlorate salts are removed by the adding reducing agents such as sodium sulfite and hydrogen sulfide to the salt solution (Japanese Patent Application Laid-Open Nos. Sho 53-123396 and Sho 6077982) or by decomposing chlorate salts in acidic conditions in the presence of an ion-exchange membrane (Japanese Patent Application Laid-Open No. Sho 63-129015). Problems associated with these methods include, respectively, the accumulation of sulfate salts and the high cost of the chemicals employed, as well as inefficiency of the decomposition of chlorate salts and difficulty in implementing the process for industrial use.
After an extensive investigation of the known methods of decomposing chlorate salts in a salt solution, applicants discovered that chlorate salts are effectively and efficiently removed from a salt solution by adding hydrochloric acid to the salt solution and then maintaining the salt solution at a reduced pressure in a decomposition vessel and/or causing the hydrochloric-acid-containing salt solution to flow upward in the decomposition vessel.